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Old 9th July 2004, 04:27 AM   #1
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Default Aperiodic box for junky drivers?

I've got some crappy generic 4" woofers that I got on buyout from Parts Express some time ago. They each have an impedance of 12 ohms, and I want to build a 6 ohm MTM center channel and some left and right speakers with these. The only problem is, the Qts of these drivers is 0.88. I remember hearing that aperiodic boxes could help lower the Qtc of sealed boxes with high Qtcs. How do I design an aperiodic box?
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Old 9th July 2004, 06:07 PM   #2
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Aperiodic - Very small boxes that "breathe" through a moving membrane. Both the membrane and cone can not be in the same exterior space. Either the membrane part has to be isolated by cutting a hole in the car so that it is outside, or the subwoofer has to be isolated from the rest of the trunk in a similar fashion to free air woofers. The "box" has to be as small as possible (ideally the membrane should be right up against the sub), since it is used only for coupling the sub and membrane. Aperiodic membrane configurations are very hard to design and tune, but give good frequency response and respond faster to transients, giving accurate and tight bass as opposed to boomy sound. They are not ruled by Thiele-Small parameters like other designs, so any woofer would work with the membrane.

- http://ccs.exl.info/frame_components.html
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Old 9th July 2004, 07:46 PM   #3
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A cross between ported and sealed providing more accuracy but less SPL.

Dynaco A25 was a highly successful aperiodic.

I think I read somewhere that they used a 5 cycle tone and stuffed until there was no more cone movement?
That could be WAY off -- my memory is bad.

Here's some more stuff...

http://www.diyaudio.com/forums/showthread/t-17089.html
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Old 9th July 2004, 10:50 PM   #4
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That's interesting how they used a 5hz signal. That means that the vent must have been pretty well stuffed. It probably doesn't work as well in a car application, where the subwoofers really pressurize the trunk to the point where an acoustic short-circuit can occur. I searched the forum and while I found lots of nice casual discussion about aperiodic venting, I didn't really see a place where someone was guided through the application of aperiodic venting to their own design. Since I don't feel like buying a Scan-Speak acoustic flow resistor, I instead plan on getting a pair of shower drain grates (like when you go in your shower to wash off and there is a drain in the floor and it's covered by a grate) and filling the space between them with some kind of material. I have three materials available to me: Parts Express open-cell damping foam, DuPont PolyFil, and fiberglass insulation material. I am thinking of trying varying amounts of each material and seeing what the result is. I am wondering, though, is a less resistive vent needed to lower the Q more, or is a more resistive vent needed? Something tells me a less resistive vent is needed, because the high Q is caused by the high compression in the box. If the high compression can be alleviated without short-circuiting the woofers (unlikely with my design where the aperiodic vent is on the back) or creating other resonances, then I would guess the aperiodic vent is doing its job. The vent can't allow too much flow or else there will be unacceptable amounts of sound output from the vent.

Do I seem to be on the right track? I guess I just have to build the things and hook them up to a 5Hz generator like Dynaco did.
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Old 9th July 2004, 11:54 PM   #5
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Sounds like a plan.

As I am sure you saw, Planet10 did what you are suggesting -- compressed fibreglass between two grates. I believe this is also exactly what Dynaco did.

Also the venerable GM uses a 'click' method? Maybe he could elaborate as he often so kindly does...

BTW -- if Planet10 is on to this there MUST be something here!
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Old 10th July 2004, 12:15 AM   #6
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...googled "Vintage Dynaco" and under Dynaco Speakers finally found the thread I was talking about

Quote:
Dynaco speakers were designed using Seas drivers, and imported to the USA under the Dynaco name. Interestingly the earliest A-25's used ScanSpeak Drivers and had the Aperiodic Port above the Tweeter, but this was soon changed to the widely seen Seas Drivers and the port under the woofer.

The production was set up that a large excursion, a 5 Hz square wave was fed into the unit, and layers of fiberglass were inserted into the body of the box, until there was minimum overshoot, which translated into maximally flat response at 50 Hz and below.

Above info courtesy Steven L. Bender.
Looks like GM's "click" method is the way to go if he is willing to share it
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Old 10th July 2004, 12:50 AM   #7
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Quote:
I've got some crappy generic 4" woofers that I got on buyout from Parts Express some time ago
Those aren't the $0.69 4" Pioneers that I just ordered are they?
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Old 10th July 2004, 01:47 AM   #8
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>Aperiodic membrane configurations are very hard to design and tune, but give good frequency response and respond faster to transients, giving accurate and tight bass as opposed to boomy sound. They are not ruled by Thiele-Small parameters like other designs, so any woofer would work with the membrane.
====
Hmm, it's been my experience that the only thing easier to design is a dipole, and T/S specs plugged into a decent box program makes it a no-brainer. The only experimental part is finding the right amount of acoustic material that matches the sim. Fortunately, if you use polyfil then it can be calc'd in MJK's Mathcad ported, TL, and ML-TQWT worksheets.

I use BoxPlot 3.0 for quick sims. Just pick a Vb and Fb, then lower Ql to suit, then find the vent area required for a baffle thickness vent, or use tube and stuff it to suit.
====
>I instead plan on getting a pair of shower drain grates (like when you go in your shower to wash off and there is a drain in the floor and it's covered by a grate) and filling the space between them with some kind of material. I have three materials available to me: Parts Express open-cell damping foam, DuPont PolyFil, and fiberglass insulation material.
====
Foam will probably overdamp it. I assume you have some of those round coffee filters close to wherever your butt's sitting . They make great aperiodic membranes, just keep stacking them as required. Another good one is fabric softener sheets. Being a cheapskate, I used the spent ones, but if you're a 'High Roller', use new ones and replace them periodically so that you can freshen your room while 'jammin'.
====
>I am wondering, though, is a less resistive vent needed to lower the Q more, or is a more resistive vent needed? Something tells me a less resistive vent is needed, because the high Q is caused by the high compression in the box.
====
More.
====
>Also the venerable GM uses a 'click' method? Maybe he could elaborate as he often so kindly does...
====
'Venerable'?! Makes me feel even older than I am. Oh well, not having any faith in what suicide bombers believe in, I figure it beats the alternative.
====
>Looks like GM's "click" method is the way to go if he is willing to share it
====
Hmm, I've posted it so many times on so many forums that I was surprised I couldn't quickly find it in a Google search. I first read about it in a '50s speaker building book and it has been reprinted in later ones as well, though I'm under the impression it goes back to the early days at Bell Labs. Here's a post I saved:

What I referred to is standing waves that occur in
pipes. These can be damped by stretching expanded
double knit, grill cloth, whatever, over the pipe's
external opening. I recommend it on any vented sub
with a pipe longer than ~24".

If you don't use a computer program to measure with,
you can determine when it's damped using this simple
ckt:

Make a tester with a s.p.d.t. toggle or pushbutton
switch, 1.5v battery and a resistor. If amplifier
damping factor is known: R = driver z/damping
factor. If not known use .5 ohm.

ON (com)
============O_______O===== Res =========== spkr +
|+
| OFF O
Battery |
| |
|- |
========================================= spkr -

(Hmm, this comes out garbled in the preview..... the
resistor side of the switch is common and OFF
position is connected to the battery/speaker (-) side.)

This circuit can also be used with an appropriate
voltage divider (say 5k over 1k for 1.5 volts) to
drive the input of your amp, to simulate the final
system accurately even with the crossover and cables
in place. No need to measure damping factor, output
impedance, or total impedance of the system.

The speaker should reproduce a distinct "click" on
'make' and 'break'. If there is "hangover" or "boom",
then stretch a piece of double knit, speaker cloth,
or similar material over the port(s) opening(s) and
test again. You may have to add more than one piece
to properly damp the speaker, but once you're there,
more just lowers LF output.

GM
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Old 10th July 2004, 04:42 AM   #9
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So, an aperiodic membrane really doesn't need to do that much? I wouldn't think a coffee filter would be up to the task. I have polyfill that I think I'd like to use. I really don't want to be swapping out coffee filters. So, I just add polyfil until the cones don't move when I give it the ol' 5 hertz? Or I could build that complicated circuit and go to a bunch of extra trouble...I think I like the Dynaco method better.

What I wonder is, I thought the open cell foam would underdamp it becuase it seems more porous to me. The simulated box with these drivers has a Q of, like, 1.05. So, wouldn't that much overdamping be needed to get the Q down into the 0.70<Qtc<0.90* range? I guess that's something I'll really have to play with. I don't know how much damping it will take to lower the Q by a set amount, but at least with my design for the aperiodic vent, I will have a wide variety of materials to play with in the 3/4" space that I have to fill with whatever.

*If I can get a lower Q without unacceptable side effects, down into the 0.5<Qtc<0.7 range, that would be cool. The 4" drivers have an excursion of 4mm and even more mechanically, and they have a Fs of 97 Hz so the drivers can basically support the excursion needed for a good transient-perfect or Bessel alignment design.
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Old 10th July 2004, 10:05 AM   #10
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'Venerable'?!
Don't l;ook a gift horse in the mouth.

Your posts -- while overly complicated and difficult to follow at times (you assume much from the reader) are of value.
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